CAGE

A 2005 Hentai Manga by Suehirogari.

Mayuko arrives at her new job to discover that her older sister Mei was hired by the same school to teach — both of them are starting up their new jobs at one of the most prestigious private schools in Japan at the same time. Unfortunately, this school for the elite has a secret — the students are training in manipulation by enslaving the school's female teachers, turning them into wanton sex slaves. Can innocent Mayuko and her slightly less innocent older sister Mei escape before the students corrupt them beyond hope?

Suehirogari followed CAGE with CAGE2 in 2006 and Ornitho in 2011, both of which are covered in the tropes below.

Tropes used in CAGE include:

A Date with Rosie Palms: One of the plots in the middle of the series involves Mei and Mayuko being forced into a set of vibrating bra and panties, which are linked -- one of the two sets will always be on -- and then sent to class. Mei had already worn a pair, due to another gambit causing her to lose her clothing when one of the students knocks her into the ocean.
Batman Gambit: Special mention goes to the heroine from Ornitho, who knew everything about the entire plot the whole time. Her friend confided to her in secret as she was being enslaved. The experience was so erotic for the heroine that she chose to transfer to the school specifically so she could join in. All of her playing dumb was due to her not wanting to scare the students and other teachers off.
Better Than It Sounds: It sounds like a simple Porn With Plot -- but then the Evil Plan kicks in.
Bi the Way: In the chapter 22 bonus chapter in the second volume, Mayuko is "rented" by one of her female students at the festival. It's played up much more tenderly than her previous sexual experiences.
Body Paint: Flat out abused in Ornitho. The female teacher gives her students the right to vote on her costume for the School Festival, with Body Paint being a joke option... She picks it (due in part to her wanting to, also due in part to her trying to think of what a blackmailed teacher who didn't know about the plan would do). At the end of the festival she loses the relay race for everyone, leaving her team stuck in their costumes for a week... but they don't have enough paint for that.
Book Ends: CAGE starts with Mei and Mayuko introducing themselves during an assembly, it ends with Mei and Mayuko looking on knowingly as the new victim introduces herself. Ornitho starts with Mutsumi's diary being narrated, it ends with the Headmistress revealing that the last segment was based on Mutsumi's diary-turned-CAGE fanfic.
Brainwashed: Mei's final fall, she was never exactly innocent (having been fired from her previous teaching position due to being caught having sex with 7 or 8 of her students), but she finally gives in after the headmistress gets her in her clutches. Upon being caught spying on the politicians having an orgy at the school with the seniors and female teachers, the headmistress and her female goons take Mei out to sea, force a vibrator upon her, which turns out to be attached to the boat's motor. Then they simply go out boating at max speed until she breaks.
[[Break the Haughty}}: Mei, who is trying to avoid any form of lust after losing her previous job due to being caught having sex with about half the male students in her class.
~~Caught with Your Pants Down~~ Caught Leaving Your Pants On The Other Side Of Town: The start of Volume 2 revolves around Mayuko (and Mei) both being suckered into trading away their clothing for hints in a game, with a student not really being sexually tortured and raped in the balance. They end up at a family restaurant just in time for their students to "discover" them standing naked in the stairway up to the restaurant. This, of course, is the ending of one and the start of another Batman Gambit, as one of the conspirators forces her to say that she did it all by herself in front of the students, and another group of students walks up in time to play Good Cop, using the event as a wedge to force her into more extreme behavior.
- Word of God (via some released images of an alternate version of the chapter) states that originally Mayuko would have been literally caught by her students as she was walking around, with someone who was observing her leaving her clothing nearby for the students who caught her to find, leaving her stuck giving the impression that she was out and about on her own. The rest of the series would have continued more or less the same, although Mei was notably absent in this version of the chapter -- her story would have diverged from this point.
Corrupt the Cutie: The entire point of the series is to train a group of politician's children in how to handle plans and manipulations by corrupting Mei and Mayuko without them realizing what's going on.
Gonk: The fat student Mei is forced to have sex with. He stinks, she can't see his penis under the flab, he doesn't give two squirts of piss about her, and the sex takes about oh... 3 seconds. Then he goes off to brag to 2ch about how he's finally lost his virginity.
If It's You It's Okay: The student couple in the later chapters. She wants to be a slave, he just wants to be with her. They compromise, putting forth the idea that they'll both be slaves, with the condition that they have to do everything together. The conspiring students go "eh, ok, sure." Last seen walking around having sex while standing upright with her hanging off his neck and walking to class.
The Masquerade: Pretty much everyone is aware of what's going on with the female teachers, especially the students "stuck" in summer school. They keep up The Masquerade to keep the teachers in the dark, in order to help the students practice their gambits.
The Mole: Seki. Given an aphrodisiac by the group, she loses a game just in time to be enslaved by the leader of the students, who is insinuated to be in love with her. By the time we see her next, she's openly going along with the plan, and helps cause Mayuko's final fall.
Mushroom Samba: Mei's fall starts when the students give her a powerful aphrodisiac, and tape her drunken confession to enjoying having sex with teenage boys. After that, she's trapped by her students. This aphrodisiac is reused again to cause another student to lose a horse race, cutting off the only ally Mayuko had amongst the student body.
- It's shown in passing that Mutsumi fell for the exact same setup in Ornitho, with her going to a welcoming party with her fellow teachers, someone jumping her in the bathroom with the aphrodisiac, and filming the results, with the students blackmailing her from there. (They give up afterwards because she's so eager it's offputting.)
Off the Rails: Ornitho's protagonist knows about the plot from the beginning, causing her to react "oddly" to the student's attempts to push her into extreme behavior. She "falls" for a blatantly obvious trick, takes it further than anyone expected, and the school headmistress has to intervene directly to solve everything once she accidentally inspires the more exhibitionistic students to join in with her.
School Festival: Two. The first is the one the students use to con Mayuko into attending a beauty contest, with the instructions of "win". In the middle of the contest, they decide to have a "bikini changing contest" -- that is, hiding the students with a hoola-hoop with towels on it and giving them 5 seconds to strip, cover themselves with a towel, and stand back up. Mayuko, drunk on the sensation of everyone looking at her, strips it off outside of the hoop and then wins the game. Later, in the epilogue, The Masquerade is completely gone and it's an openly sexualized party... only for the conspiratorial girls to challenge Mayuko to basketball, not knowing she was a basketball star in high school. She beats them handily, forcing them to experience a taste of their own sexual torture.
Sequel Hook: The final chapter has Mayuko and Mei quietly watching while another new teacher is introducing herself to the student body, just like how Mayuko and Mei did. This teacher's story is told in the sequel, Ornitho.
Sibling Yin-Yang: Mayuko is a conservative, shy virgin who is almost genki with her enthusiasm and purity at times. Mei is a laid back lecher who was fired from her previous job after being caught having sex with 8 different male high school students. Or was she hired by a private eye to infiltrate the school with a very good cover story and discover just why so many government officials visit the school? Either way, both of them don't end up the same by the end.
Spell My Name with an "S": The sequel's name is オルニト, which directly reads as "Orunito." However, publicity shots of the omnibus cover show the correct Westernized reading is Ornitho, which is Greek for "bird."
Unusually Uninteresting Sight: Even as the Mei and Mayuko do more and more obscene things in public, the students pay less and less attention to them. This is partially due to The Masquerade being abandoned -- the students have seen this kind of thing for years. For example, by the end of the series, Mayuko has given away every scrap of clothing she owns (leaving her walking around nude 24x7), Mei is walking around wearing wearing the kinky dress from The Fifth Element (without underwear), having sex with random people; a pair of students are training so they can have sex while walking around (both of them eschew pants in order to make this possible); and no one cares.
- This is dialed Up to Eleven in Ornitho: Mutsumi ruins the timeline by going naked to school before summer school begins, which inspires some of her students to go naked too, the peer pressure of which pushes other students into joining in. Unlike with the teachers, the sight of other students walking around naked is hard to ignore by the other students -- they're shown to be obviously distracted. The headmistress has to intervene around the time half the female student body is walking around nude; to curb abuses she institutes a new policy: The campus is clothing optional during summer term, but anyone going naked isn't allowed to refuse anyone's advances.
What Could Have Been/Cut Short: The series as a whole seems to have been intended to be quite longer, with several new characters introduced in chapters 20/21/22 (including several new enslaved teachers, and a pair of self-enslaved students) and Mei (and the subplot about the detective/reporter) falling Out of Focus for about half the series.
- Part of this is being handled by Ornitho — Ornitho covers the teacher that shows up at the end of CAGE 2, Ornitho 2 covers the new teachers mentioned towards the end of CAGE 2.
- This has been confirmed by Word of God: A leaked version of chapter 14's big plot twist was found, wherein Mayuko was caught by her students on the side of the road instead of getting caught going into a restaurant, with Mei nowhere in sight. Mei's story would have diverged significantly from this point, whereas Mayuko's would have continued pretty much as it did before.
Xanatos Gambit: The school turns out to be specifically set up to train the students of the elite in how to plan and manipulate in the form of enslaving their female teachers without them catching on to what's going on. The specific benchmark is stated in Ornitho — getting teachers to attend class in the nude without them realizing they're being manipulated into doing so. It's also shown by the end that the enslaved teachers and students are used as prostitutes by the local government officials (who are all alumni of the school).
- In other words, those that learn fulfill the purpose of the school. Those that don't fulfill the pleasure of those that started the school. Both benefit the planners.